1. Field of the Invention
This invention relates to the field of color characterization, and specifically relates to transforming a device-dependent color value in a device-dependent color space of a display device to a device-independent color value in a device-independent color space, wherein the transformation accounts for the channel interdependency prevalent in digital light processing (DLP) technology.
2. Description of the Related Art
Typically, display devices such as liquid crystal displays (LCDs) and cathode-ray tubes (CRTs) produce colors on a screen by combining different amounts of red, blue and green (RGB) light. The actual color produced from specific combinations of RGB light varies from display device to display device. Therefore, in order to consistently reproduce colors on different devices, manufacturers of display devices create device profiles that characterize the output of a particular display device in terms of a standard color coordinate system. For instance, the CIEXYZ color space is often used.
Display devices based on CRT or LCD technology typically exhibit channel independence, in which each of the RGB channels function independently or nearly independently of each other for color characterization. Because they exhibit channel independence, CRT and LCD display devices have been successfully modeled by a so-called “standard matrix model.” In the standard matrix model, RGB values are mapped to XYZ values by adjusting the RGB values according to generally non-linear tonal values (tone curves) and applying a tristimulus matrix to the adjusted RGB values, as follows:
                    ⁢                  R        ′            =                        γ          r                ⁡                  (          R          )                                G      ′        =                  γ        g            ⁡              (        G        )                        B      ′        =                  γ        b            ⁡              (        B        )                        (                                    X                                                Y                                                Z                              )        =                  S        ⁡                  (                                                                      R                  ′                                                                                                      G                  ′                                                                                                      B                  ′                                                              )                    +              (                                                            X                bkpt                                                                                        Y                bkpt                                                                                        Z                bkpt                                                    )            where R′, G′ and B′ represent radiometric scalars obtained by transfer functions γr(R), γg(G) and γb(B), and where Xbkpt, Ybkpt and Zbkpt are the components of the XYZ measurement of the blackpoint. S represents a tristimulus matrix as follows:
  S  =      (                                        X                          r              ,              max                        c                                                X                          g              ,              max                        c                                                X                          b              ,              max                        c                                                            Y                          r              ,              max                        c                                                Y                          g              ,              max                        c                                                Y                          b              ,              max                        c                                                            Z                          r              ,              max                        c                                                Z                          g              ,              max                        c                                                Z                          b              ,              max                        c                                )  where superscript c represents a black-corrected measurement, where subscripts r, g, and b denote the channel, and max denotes a color ramp point with a maximum device value. The standard matrix model assumes channel independence, and therefore this model is a good fit for characterizing CRT and LCD display devices.
Display devices using DLP technology, however, exhibit channel interdependency, in which the RGB channels do not function independently of each other. There are two primary reasons why channel independence fails for such devices. First, color management in the form of nonlinear 3D look-up-table (LUT) may be implemented in the hardware of the DLP display device in order to overcome artifacts (e.g. Abney Effect) due to the response of the human visual system. In addition, in the case where a white filter is present in the DLP display device, an internal fourth (white) channel is generated to enhance the luminance significantly in addition to the contribution from the RGB channels, thus invalidating an additivity assumption that would be a consequence of channel independence.
Since DLP devices exhibit channel interdependency, and since the standard matrix model assumes channel independence, the standard matrix model alone does not adequately characterize DLP display devices.
In addition to the standard matrix model, other models have been developed for characterizing display devices. For example, one model uses an intercepting LUT between the tone curves and the tristimulus matrix in order to model channel interaction of display devices that exhibit channel interdependency (see Woolfe, Geoff J. et al, an Improved Method for CRT Characterization Based on Spectral Data, Proceedings of CIE Expert Symposium 1997 on Colour Standards for Imaging Technology). However, this model fails to consider the effect of a white filter often present in DLP display devices. Another model applies a white scalar to compensate for the presence of a white filter, but this model does not necessarily account for the channel interdependency found in DLP display devices (see Wyble, David R. and Zhang, Hongqin, Colorimetric Characterization Model for DLP Projectors, Proceedings of IS&T/SID Eleventh Color Imaging Conference).